The barrier to radial oxygen loss protects roots against hydrogen sulphide intrusion and its toxic effect.

The root barrier to radial O loss (ROL) is a key root trait preventing O loss from roots to anoxic soils, thereby enabling root growth into anoxic, flooded soils. We hypothesized that the ROL barrier can also prevent intrusion of hydrogen sulphide (H S), a potent phytotoxin in flooded soils. Using H S- and O -sensitive microsensors, we measured the apparent permeance to H S of rice roots, tested whether restricted H S intrusion reduced its adverse effects on root respiration, and whether H S could induce the formation of a ROL barrier. The ROL barrier reduced apparent permeance to H S by almost 99%, greatly restricting H S intrusion. The ROL barrier acted as a shield towards H S; O consumption in roots with a ROL barrier remained unaffected at high H S concentration (500 μM), compared to a 67% decline in roots without a barrier. Importantly, low H S concentrations induced the formation of a ROL barrier. In conclusion, the ROL barrier plays a key role in protecting against H S intrusion, and H S can act as an environmental signalling molecule for the induction of the barrier. This study demonstrates the multiple functions of the suberized/lignified outer part of the rice root beyond that of restricting ROL.